This invention relates to processes for the production of a gas containing hydrogen and carbon oxides (such as methanol synthesis gas) by steam reforming a hydrocarbon feedstock, and in particular to an apparatus and method for hydrocarbon reforming processes which utilize high grade sensible heat of flue gas and product synthesis gas to generate additional product gas and minimize steam export.
The steam reforming process is a well known chemical process for hydrocarbon reforming. A hydrocarbon and steam mixture (a “mixed-feed”) reacts in the presence of a catalyst to form hydrogen, carbon monoxide and carbon dioxide. Since the reforming reaction is strongly endothermic, heat must be supplied to the reactant mixture, such as by heating the tubes in a furnace or reformer. The amount of reforming achieved depends on the temperature of the gas leaving the catalyst; exit temperatures in the range 700°-900° C. are typical for conventional hydrocarbon reforming.
Conventional catalyst steam reformer processes combust fuel to provide the energy required for the reforming reaction. In a reformer of such a conventional process, fuel typically is fired co-current to incoming cold feed gas to maximize heat flux through the tube wall(s) by radiant heat transfer directly from the flame. Downstream from the burner end, both the product gas and the flue gas exit at relatively high temperatures. The energy content of these gases usually is recovered by preheating reformer feed gas or by generating steam. As a result, the process generates excess steam that must be exported to improve the overall efficiency of the steam reforming process and to make the process economically feasible in view of the fact that significant equipment has been added to generate that excess steam.
Each of the processes disclosed in U.S. Pat. No. 5,199,961 (Ohsaki, et al.) and U.S. Pat. No. 4,830,834 (Stahl, et al.) and in European Pat. No. EP 0 911 076 A1 (Stahl) utilize a portion of the available sensible energy within the reformer vessel, thereby allowing the product gas and the flue gas to exit at lower temperatures than the corresponding exit temperatures for conventional steam reforming. These reforming processes receive heat from the combustible fuel by using a combination of: (1) an equalizing wall (made of tiles, refractory, or metals) to receive radiant heat directly from the flame from which heat is transferred to the reformer tube(s) by radiant heat; and (2) an arrangement of a counter-current flow of the hot flue gas with the incoming feed which transfers sensible energy to the incoming feed by convection through the tube wall. These techniques allow the temperature of the reformer tube skin to be controlled within the design limit; otherwise, the temperature will be excessive due to the high intensive radiant heat of the flame. However, these processes suffer a heat flux limitation by avoiding the direct radiant heat from the flame to the tube as commonly used in conventional reformers.
U.S. Pat. No. 5,945,074 (Waterreus, et al.) discloses tunnels to remove combustion product gases from a combustion chamber. The tunnels serve to balance or maintain uniform flow of the combustion gases through a furnace but do not utilize the high temperature sensible heat of the combustion gas.
To recover the sensible heat of product gas, prior art hydrocarbon reforming processes use a tube within a tube (tube-in-tube) arrangement with catalyst in the annuli. The cold feed in the annuli flows counter-current with the combustion or the flue gas from the outside and absorbs the combustion heat of both the radiant and convection heat transfers through the outside tube wall. The reformed gas flow is reversed at the end of the catalyst bed and enters the inner-most passage of the tube. The reformed gas then gives up heat to the counter-current flow of the incoming cold feed. The convection heat transfer process from the hot product gas to the reforming reactions is not effective however, because there is no temperature driving force at the inversion point. As a result, more heat transfer surface area is required to utilize the product gas sensible heat. Consequently, the lack of intensive radiant heat transfer from the outside and the ineffective convection heat transfer in the inside result in a large tube-in-tube requirement.
It is desired to have an apparatus and a method for hydrocarbon reforming processes which overcome the difficulties, problems, limitations, disadvantages and deficiencies of the prior art to provide better and more advantageous results.
It is further desired to have an apparatus and a method for a hydrocarbon reforming process which utilize high grade sensible heat of flue gas and product synthesis gas to generate additional product gas and minimize steam export.
It is still further desired to have a more efficient and economic process and apparatus for hydrocarbon reforming.